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unix server is up or not using ssh

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# 1  
Old 10-05-2012
unix server is up or not using ssh
Hi,
I have three servers like S1,S2 and S3
I want to check the S2 and S3 server status from S1 using ssh method.
Code:
ssh user@S2 date >> test.txt
ssh user@S3 date >> test.txt

Am using the above format to get the details. But if the server is down, The code has been hanged and not coming out of it.
Please suggest on this, how to handle.

Thanks,
Chelladurai Smilie
Last edited by vbe; 10-05-2012 at 12:50 PM.. Reason: code tags
# 2  
Old 10-05-2012
I've especially seen ssh hang when the server is up but barely responding. You can put it in the background and wait up to a maximum amount of time before killing it. Your system will need to have /proc/.

Code:
ssh username@host date </dev/null 2>/dev/null >logfile &
PID=$!

TRIES=0
while [ "$TRIES" -lt 10 ]
do
        [ -d "/proc/$PID" ] || break
        sleep 1
        let TRIES=TRIES+1
done

# If it still exists despite timeout, first kill it properly, and if it refuses to die, kill it mean
[ -d /proc/$PID ] && kill -TERM "$PID" && sleep 1 && [ -d /proc/$PID ] && kill -9 /proc/$PID

wait

Last edited by Corona688; 10-05-2012 at 12:46 PM..
# 3  
Old 10-05-2012
Code:
ssh -o ConnectTimeout=1 user@S2 date >> test.txt
ssh -o ConnectTimeout=1 user@S3 date >> test.txt

# 4  
Old 10-05-2012
Quote:
Originally Posted by in2nix4life
Code:
ssh -o ConnectTimeout=1 user@S2 date >> test.txt
ssh -o ConnectTimeout=1 user@S3 date >> test.txt

This can still hang if the server is slow responding for other reasons, like swapdeath -- you might get in but hang afterwards, ssh won't kill itself when that happens.

Not all ssh complies with -o options either.
# 5  
Old 10-09-2012
Hi Corona,
Could you please the usage of the following varibale in the script.

1) PID=$!
2)[ -d "/proc/$PID" ] || break


How it works? .

Thanks in advance .
# 6  
Old 10-09-2012
PID=$! assigns the process ID of the most recently executed background command to the variable PID and thats the ssh-command.
[ -d "/proc/$PID" ] || break checks if there exists a directory in /proc with the name of your process ID. Each process usually has such a directory on most *nix-like OSes.
Edit: the || break part says leave the loop if there is no such directory anymore.

Btw, with your method you check if the sshd demon of the server is up and running. That demon being down does not neccessarily mean the whole server is down.
Last edited by cero; 10-09-2012 at 07:14 AM..
# 7  
Old 10-09-2012
Thanks for explaining Cero.
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